There has been known a hybrid vehicle drive apparatus including an engine, a first electric motor, a second electric motor and a planetary gear mechanism which has a first rotary element connected to the engine, a second rotary element connected to the first electric motor and a third rotary connected to a power transmitting path connected to drive wheels, the second electric motor being connected to the power transmitting path. Such a hybrid vehicle drive apparatus is disclosed in, for instance, FIG. 1 of Patent Publication 1 (Japanese Patent Application Publication No. 2006-242096). With such a hybrid vehicle drive apparatus, one of possible tasks was to minimize rattling noise of gears, constituting the power transmitting path, which occurred during a start-up or a halt of the engine.
With a control device of the hybrid vehicle drive apparatus disclosed in Patent Publication 1, the gears are maintained in a rattling-free connection with the use of an output torque of the second electric motor. With the engine having no start-up request, if output torque of the second electric motor exceeds a given value, then, the engine is started up. In contrast, if output torque of the second electric motor is less than the given value, then, a determination is made that the rattling-free connection is inadequate. In this case, the engine stop operation is continuously conducted in line with conditions under which demanded torque of a driver is less than a given value, and a battery has a remnant state of charge SOC exceeding a given value. Further, if demanded torque exceeds the given value and the remnant state of charge SOC is less than the given value, the engine is started up regardless of the degree of the rattling-free connection.
Besides, there have been known other technologies as disclosed in Patent Publication 2 (Japanese Patent Application Publication No. 11-93725), Patent Publication 3 (Japanese Patent Application Publication No. 11-173171), Patent Publication 4 (Japanese Patent Application Publication No. 2006-105386) and Patent Publication 5 (Japanese Patent Application Publication No. 2006-242095).
With the control device of the hybrid vehicle drive apparatus disclosed in Patent Publication 1, it was likely that the engine start-up was initiated that is the engine is started up without achieving a reduction in rattling noise of the gears under various situations. These situations arise in: a first case wherein output torque of the second electric motor is less than a given value and demanded torque is less than a given value while the remnant state of charge of the battery is less than a given value; a second case wherein a determination is made based on multiple conditions and the engine start-up was unsuccessful; or a third case wherein an attempt is made to forcibly initiate the engine start-up regardless of whether the rattling-free connection is adequate.